INTRODUCTION: Physical therapy during phase I of cardiac rehabilitation (CPT) can be started 12 to 24 hours after acute myocardial infarction (AMI), however, it is common to extend the bed rest due to fear of patient’s instability. OBJECTIVES: To assess the hemodynamic and autonomic responses to post-AMI patients when subjected to first day of phase I protocol of CPT, as well as their safety. MATERIALS AND METHODS: We studied 51 patients with first uncomplicated AMI, 55 ± 11 years, 76% men. The patients were subjected to first day protocol phase I CPT, on average, 24 hours after AMI. The Instantaneous heart rate (HR) and RR interval were acquired by HR monitor (Polar™S810i) and blood pressure (BP) checked by auscultation. HR variability was analyzed in the time (RMSSD and RMSM-Ri in ms) and frequency domains. Power spectral density was expressed in absolute (ms2/Hz) and normalized (nu) units for the bands of low (LF) and high frequencies (HF) and as LF/HF ratio. RESULTS: The RMSSD, HF and HFnu have reduced performance of the exercises in relation to rest and post-exercise (p < 0.05), LFnu and LF/HF ratio increased (p < 0.05). HR and systolic BP showed an increase during the execution of the exercises in relation to rest (p < 0.05). There were no any signs and/or symptoms of exercise intolerance. CONCLUSION: The exercise was effective, because it caused changes hemodynamic and autonomic modulation in these patients, without causing any medical complications.